Vascular disease is one of the most common diseases worldwide and, with the increasing age and longevity of the American population, will remain a major public health priority (Conte, 1998). The mainstay of therapy for patients with coronary and peripheral occlusions is surgical bypass grafting with the patient's own (autograft) saphenous vein or internal mammary artery. This procedure offers benefits to the patient but incurs significant economic cost on the national level. Commonly, insufficient autograft material is available and a patient may receive cadaver tissue (allograft). However, allograft conduits have poor patency rates resulting in numerous revision surgeries, amplifying the economic impact. A reduction in graft patency is a result of an immunological response to cellular material contained within and on the allograft leading to intimal hyperplasia and eventual graft occlusion.
Allografts can have complications leading to early stenosis and graft failure thought to be related to an immune reaction similar to graft rejection. Adaptation of vein allografts to the arterial environment has been studied extensively in animal models and also in humans, to a lesser degree (Conte, 1998). After implantation, the conduits undergo structural changes characterized by intimal hyperplasia and overall wall thickening (Conte, 1998). Cellular events that occur after implantation can affect the patency by occluding the conduit. Occlusion or stenosis in coronary or peripheral circulation is a common clinical occurrence present in small-caliber applications that necessitates an additional intervention (Conte, 1998). Problems that cause occlusion of a graft are thrombosis and neointimal hyperplasia (Conklin, 2002; DeMasi, 1995). Thrombosis occurs when platelets in circulating blood adhere to certain surfaces, then release chemicals to attract more platelets to form a large aggregate that generates thrombin (Stillman). Grafts most commonly fail due to the development of fibrous intimal hyperplasia where there is an excess proliferation of smooth muscle cells. This flow-restricting lesion may occur diffusely throughout the graft or, more commonly, at focal sites near anastomoses, particularly in compliance mismatched synthetic grafts (Conte, 1998).
Current processing methods only treat the tissue without removing a substantial amount of endogenous material that may harbor contaminants, such as blood and lipids, as well as antigen containing cells and cellular debris that elicit an immune reaction. Processing methods and cryopreservation preserve and retain endothelial cells and smooth muscle cells (SMC), both containing antigens that may require matching blood types for vascular reconstructions. Patients in need of an allograft must wait for a matching donor, and the waiting time for a compatible (ABO relevant) graft depends on the rarity of the recipient's blood type (Prager, 2002).
Rejection plays a significant role in failure and leads to allosensitization (Timaran, 2002). Therefore, a great deal of effort is placed in determining an ABO-compatible donor as well as lymphocyte cross-matching (Prager, 2002). The intimal layer of the conduit contains the antigen-present endothelial cells responsible for ABO relevancy. Stripping this layer of cells from the conduit removes the need for donor matching, but it is believed by some that a lack of surviving cells and endothelial integrity may play a role in graft degeneration and early and late patency (Prager, 2002). If injured and subconfluent endothelial cells line the lumen, thrombosis and SMC growth is promoted leading to intimal hyperplasia (Bader, 2000). Thus there is a need for a process which decellularizes grafts to eliminate the need for blood-typing, reduce platelet activation, and prevent smooth muscle cell proliferation that leads to neointimal hyperplasia resulting in thrombosis.
In the case of multiple revisions, insufficient autograft material, or infection due to synthetic material, a vascular allograft or xenograft is a medical necessity for life and/or limb saving operations. Currently, the demand for vascular allografts far exceeds the supply. Allograft tissue is generally obtained from cadavers and processed under aseptic conditions. Current processing methods such as using an antimicrobial soak can result in a substantial discard rate due to positive first and final cultures from bacterial and fungal contamination resistant to the cocktail aimed at achieving sterility. Additionally, the risk of transmission of viruses and other pathogens can be reduced but not eliminated through donor screening. Therefore, vascular transplantation carries some risk of disease transmission to the recipient considering the occurrence of false negative test results, human errors in screening and processing, and virological testing within the window period of contraction and detection of diseases. Over the last several years multiple examples of donor to host disease transmission have been documented. These examples include transmission of hepatitis C, bacteria, and fungi, and have resulted in serious morbidity and even mortality (Kainer, 2002). Traditional methods of sterilization, such as irradiation and ethylene oxide, have significant deleterious effects on tissue integrity that limit or preclude their use on allograft tissue.
Surgeons acknowledge that there are inherent risks in dealing with human tissue and tend to first use autograft material (DeMasi, 1995). However, patients who do not have sufficient or acceptable autografts, or are unable to receive synthetic grafts are must use allografts. To that end, the FDA has taken a recent interest in how human tissue might be held to a higher standard (Administration, 2004). This would increase the availability of allograft by reducing the discard rate and provide a safe alternative to autograft.
Factors that indicate a successful graft include one or more of the following: no aneurysms or dilations, no immunological reaction, no disease transmission, no infection and long-term patency (Bastounis, 1999; Benedetti-Valentini, 1996; DeLaurentis, 1971; Lord, 1975; Oppat, 1999). Once placed in arterial circulation, it must be capable of withstanding long-term hemodynamic stress without mechanical failure. A failure of this type could be catastrophic and lead to morbidity, such as loss of limb, or even mortality. The availability, suturability, and simplicity of handling for a graft are desirable for minimizing operating time, risk and expense as well as long-term durability. Postimplantation, the graft should be fully biocompatible, resistant to thrombosis and infection and be completely incorporated by the body to yield a neovessel resembling the native artery in structure and function (Conte, 1998; Timaran, 2002). The graft should be porous enough to permit ingrowth of tissue, but still durable and suitable to maintain anastomosis integrity.
In view of the foregoing considerations, there has been a long felt need for vascular grafts that meet the above criteria. It is an object of this invention to provide a process that creates an inert scaffold out of allograft or xenograft veins (e.g. human saphenous veins). By removal of endogenous materials the antigenicity of the graft is reduced and recipient cell repopulation is promoted following implantation, bypassing the initial graft rejection response typically seen in allograft or xenograft use. It is an additional object to provide sterile grafts by achieving an acceptable reduction in harmful organisms.